The basic structure and operation of a cellular telephone communications system has been disclosed in a variety of publications. See, for example, the January 1979 issue of The Bell System Technical Journal; and Specification EIA IS-3-B (as amended) entitled "Cellular System Mobile Station-Land Station Compatibility Specification" (July, 1984, Electronic Industries Association)
As is well known, the process called "handoff" is a fundamental part of the cellular radio telephone scheme. The following publications are generally relevant in disclosing handoff techniques:
Huensch et al: U.S. Pat. No. 4,475,010: OCT 1984 PA0 Ito et al: U.S. Pat. No. 4,144,412: MAR 1979 PA0 Kojima et al: U.S. Pat. No. 4,435,840: MAR 1984 PA0 Cunningham et al: U.S. Pat. No. 4,144,496: MAR 1979 PA0 Cox et al: U.S. Pat. No. 3,764,915: OCT 1973 PA0 Leland: U.S. Pat. No. 4,384,362: MAY 1983 PA0 Frenkiel: U.S. Pat. No. 4,144,411: MAR 1979 PA0 Brody et al, "Application of Digital Switching In A Cellular Mobile Radio System", International Switching Symposium (May 7-11, 1984, Florence, Italy) PA0 Ma et al, "DMS-MTX Turnkey System For Cellular Mobile Radio Application", IEEE 1984 Vehicular Technology Conf., Pittsburgh, PA. (May 1974)
A brief description of the handoff process will now be presented. A simplified cellular radio telephone system 10 is shown in FIG. 1. Cellular system 10 includes several fixed RF transceiving stations 12 each serving an associated discrete geographical area ("cell") 14 (a simplified overlapping 7-cell system is shown). A central controller 16 ("MTX") supervises and controls the operation of fixed stations 12. Assume a mobile transceiver 18 has an initial position within cell 14A, but is now moving out of that cell into cell
Central controller 16 continuously determines the location of mobile transceiver 18 by requesting stationary transceiver 12A to measure the received signal strength of the mobile transceiver transmissions. When central controller 16 (or stationary transceiver 12A) determines, based on signal strength measurements, that mobile transceiver 18 has moved too far away from stationary transceiver 12A for high-quality communication to be maintained, the central controller attempts to determine which cell the mobile transceiver is moving into (i.e., by requesting all of stationary transceivers 12A-12G associated with cells adjacent to cell 14A to measure mobile transmission signal strength) Central controller 16 then selects the cell with the highest received signal strength as the cell the mobile transceiver is most likely moving into.
The central controller controls the fixed station 12A serving the first cell 14A to discontinue handling the mobile station radio call and controls the fixed station 12B (into which the mobile transceiver is moving) to begin handling the call (and also controls the mobile station to retune to a frequency fixed station 12B operates on). In this way, the mobile station 18 (and its call) is "handed off" to the cell receiving the strongest signal from the mobile station. High quality uninterrupted communication is thus maintained even while mobile station 18 is moving from one cell to another.
System 10 typically measures the RF signal strength of transmissions of mobile station 18 at the locations of fixed stations 12 in order to decide when a handoff should occur and to what cells calls should be handed off to. Decreased received signal strength at a fixed station 12 handling the call of mobile station 18 indicates that the mobile station is nearing the edge of the cell 14 served by the fixed station and is likely to need handing off to another cell. Signal strength measurements performed by fixed station 12 serving adjacent cells are used to determine which cell the call should be handed off to, thus maximizing communications quality and reliability and minimizing the number of handoffs necessary.
When system design includes partitioned cells (pie-shaped sectors, overlayed cells, etc.), signal strength measurements at fixed stations may also be used to determine which cell partition may best serve particular mobile stations.
As will be appreciated, signal strength measurements are very important in the design and operation of cellular radio telephone communications systems, and are indeed an essential requirement of cellular equipment design.
Every handoff in a cellular radio telephone system requires a number of signal strength measurements. Since cellular systems typically serve large numbers of mobile stations, hundreds of signal strength measurements may be required every few minutes. Moreover, because mobile stations are usually in motion, the cellular system must respond very rapidly to changes in received signal strength (e.g., by handing off calls) to maintain acceptable signal levels as mobile stations move from cell to cell. There is therefore a great need for fast and accurate received signal strength measuring techniques.
RF signals transmitted by mobile radio stations are subject to Rayleigh and Gaussian fading, as is well known. Such fades are of short duration and may be 20 dB or more below the average received signal strength level, making it difficult to obtain accurate and rapid signal strength measurements (since a measurement made during a deep fade is not representative of the true average received signal strength of a mobile radio transmission).
Prior art methods of overcoming this difficulty include analog filtering (equivalent to damping a meter movement so that it does not respond to fast transients), and mathematically averaging several samples of received signal strength measured by a given locating receiver. Such prior art techniques require several measurements to be taken over a period of time large enough to mask the effects of fading. Unfortunately, the extended time period required to obtain accurate measurements using such techniques is in conflict with the requirement that received signal strength measurements must be made as rapidly as possible.
As mentioned previously, in the prior art, field strength of signals transmitted by a mobile transceiver during an ongoing call is continuously monitored at the cell site serving the call. If the signal received from the mobile transceiver becomes weak, it is assumed that the mobile transceiver is at cell boundary, and that a handoff is necessary. The cell site controller generates a list of candidate cells (usually cells which neighbor or overlap the serving cell) and commands those candidate cells to measure the signal strength of the mobile transceiver. The cell receiving the strongest signal is assumed to be the cell into which the mobile transceiver is entering, and the call is handed off to that cell.
Sometimes, however, the cell receiving the strongest signal strength is not the best cell to handle the call. For example, RSSI measurements are sometimes made while the mobile transmissions are obstructed (e.g., when the mobile transceiver is on or beneath a bridge, is "shadowed" by a tall building, or passes behind a hill or into a valley). The call may be handed off to a "wrong" cell (i.e., a cell which is physically distant from the mobile transceiver location relative to other, closer cells) because the decision as to which cell is the "best" candidate for handoff must be determined in a relatively short time (to guarantee fast system response time).
For instance, a typical cellular system may be required to make a signal strength measurement over a period of only 40 milliseconds. Due to obstructions in the signal path (e.g., trees and buildings), the field strength measured over such a short period of time can deviate by 12 dB or more from the average field strength for a mobile transceiver in that area.
To improve measurement accuracy, it is necessary to average several field strength measurements acquired over a period of 1 to 10 seconds. Unfortunately, this approach increases response time and system cost. Locating receivers must measure the signal strength of many channels, and are therefore generally busy all of the time (at least during peak hours of cellular system usage). An increase in measuring time requires additional locating receivers and associated peripheral circuits to distribute the field strength measurement work load. If measurement time is made too long, system response time is degraded--and ongoing calls may be lost before a handoff decision is made.
Handoffs to incorrect cells are of concern but are not a serious problem in cellular systems which do not require frequency reuse. Incorrect handoffs increase message traffic (since a call handed off to an incorrect cell will most likely soon have to be handed off to the correct cell), but do not generally result in lost calls or unacceptable service--since the selected handoff candidate, even if it is not the best candidate, may still be guaranteed to provide acceptable service to the mobile transceiver, at least for a short period of time.
Unfortunately, erroneous handoff decisions may produce severe problems in mature cellular systems using frequency reuse. In such mature systems, the same radio operating frequency may be in use simultaneously within several cells of the system (prescribed distances are maintained between co-channel cells to prevent co-channel interference). The potential for co-channel interference is increased if a call to a mobile transceiver is handled by a cell which is physically distant from the mobile transceiver location, since the mobile transceiver in such a case will almost always be operating on a frequency which is not assigned to the cell it is located within. The mobile transceiver, being physically distant from the cell site assigned to use the operating frequency, may be close enough to co-channel cells to cause co-channel interference. In extreme cases, one co-channel call may "block" another, causing the blocked call to be dropped, and resulting in an unhappy customer.
Cell sectorization has been suggested by others as a way to reduce the number of handoffs to incorrect cells, and thus, avoid co-channel interference problems in mature cellular systems Yet, cell sectorization only somewhat reduces and does not eliminate incorrect handoffs. Accordingly, there is a great need for a technique which is applicable to any type of cellular system (omni-directional or sectored) which reduces the number of incorrect handoff decisions without increasing equipment requirements or received signal strength measuring time.